In "Seymour: an Introduction" Buddy discusses at length "the four variously notorious Sick Men," four guys he runs to when he wants "perfectly credible information about modern artistic processes" ("Seymour" 1.2). The men on this list are: Søren Kierkegaard, a 19th century Danish philosopher and the so-called "father of Existentialism" (though Buddy finds it amusing that Kierkegaard was, in fact, not an existentialist himself); Franz Kafka, a 20th century writer; Vincent van Gogh, a 19th century Dutch artist, and of course Seymour himself.
All these men were great artists (either prolific writers or painters). According to Buddy, they were also "sick" in one way or another. Kierkegaard was devoutly religious to the point of psychological self-torture; Kafka suffered from a number of ailments, including depression, social anxiety, and insomnia; van Gogh famously cut off part of his left ear, suffered from depression, and killed himself at the age of 37; and Seymour, as we know, suffered from what today we might call Posttraumatic Stress Disorder, and eventually committed suicide.
In a syntactically convoluted, long page paragraph at the start of "Seymour," Buddy explores just what it means to be a sick man and an artist. It's a section worth reading a second (or fifth) time. At least take a look at least this excerpt here:
But what […] one most recurrently hears about the curiously-productive-though-ailing poet or painter is that he is invariably a kind of super-size but unmistakably 'classical' neurotic, an aberrant who only occasionally, and never deeply, wishes to surrender his aberration; or, in English, a Sick Man who not at all seldom […] gives out terrible cries of pain, as if he would wholeheartedly let go both his art and his soul to experience what passes in other people for wellness, and yet […] when his unsalutary-looking little room is broken into and someone […] asks him where the pain is, he either declines or seems unable to discuss it at any constructive clinical length, and […] looks more perversely determined than ever to see his sickness run its course, as though […] he had remembered that all men […] eventually die, […] but that he, […] is at least being done in by the most stimulating companion […] he has ever known. ("Seymour" 1.2)
The concept of the sick artist is an interesting one. So often, this passage reminds us, the world's greatest artists are also sick men – unhappy, tortured, troubled men. There's a beautiful line in another of Salinger's short stories, "De Daumier-Smith's Blue Period," in which a painter says to an aspiring student: "The worst that being an artist could do to you would be that it would make you slightly unhappy constantly." He then adds, "However, this is not a tragic situation, in my opinion."
According to Salinger, then, unhappiness is the price one pays for artistry. This leads to some interesting questions about Buddy's repeated insistence that he is happy, ecstatic even, while narrating Seymour. Perhaps this relates to his frustration with his own writing – how can be a true artist when he is so happy?
But what exactly tortures the sick artist? What makes him sick? Buddy takes a stab at this question and makes a decent argument as to the source of the sick artist's pain:
But where does by far the bulk, the whole ambulance load, of pain really come from? Where must it come from? Isn't the true poet or painter a seer? Isn't he, actually, the only seer we have on earth? […] In a seer, what part of the human anatomy would necessarily be required to take the most abuse? The eyes, certainly. Please, dear general reader, as a last indulgence (if you're still here), re-read those two short passages from Kafka and Kierkegaard I started out with. Isn't it clear? Don't those cries come straight from the eyes? However contradictory the coroner's report - whether he pronounces Consumption or Loneliness or Suicide to be the cause of death - isn't it plain how the true artist-seer actually dies? I say […] that the true artist-seer, the heavenly fool who can and does produce beauty, is mainly dazzled to death by his own scruples, the blinding shapes and colors of his own sacred human conscience. ("Seymour" 1.2)
We can start to see why Seymour – a "God-knower" who actually sees more than the rest of us (see "Characterization") – belongs on this list of Sick Men.